The present invention relates to an image output method and apparatus and, more particularly, to an image output method and apparatus suitable for performing image output by, e.g., an ink-jet system.
Generally, an image output apparatus such as a printer, a copying machine, and a facsimile apparatus records, as an output image, an image consisting of a dot pattern on a recording medium such as a paper sheet or a plastic thin plate on the basis of input image information.
Such image output apparatuses can be classified into an ink-jet system, a wire-dot system, a thermal system, a laser beam system, and the like depending on their recording methods. Of these systems, the ink-jet system (ink-jet printer) ejects and flies ink droplets (recording liquid) from the ejection orifices of a recording head and sticks them to a recording medium, thereby recording an image.
Along with the recent spread of the image output apparatuses, there have been strong needs for high-speed recording, high resolution, high image quality, low noise, and the like. The ink-jet printer is an image output apparatus meeting these requirements.
As one of values added, a high image quality has been required in the image output apparatus of an electrophotographic system in recent years, and various smoothing techniques have been developed. To perform smoothing, basically, the resolution of all image data can be increased. However, when the resolution is simply increased, the number of data is largely increased.
For this reason, a conventional method is mainly used in which a target smoothing portion is extracted from image data with a resolution of, e.g., 300 DPI, and only the extracted portion is interpolated and drawn at a dot density of, e.g., 600 or 1,200 DPI.
The above method is disclosed in U.S. Pat. Nos. 4,544,264, 2,625,222, 4,933,689, and the like.
In U.S. Pat. Nos. 4,544,264 and the like, the prior-art invention has been made on the basis of the characteristics of the electrophotographic technique. More specifically, one of basic smoothing techniques, the resolution of raster data is increased. A polygon mirror is rotated at a higher speed than a normal speed, and the density of a raster interval is increased, thereby obtaining a higher resolution of the raster data. Smoothing is performed by using this high resolution and adding dots having a medium size or density.
However, the conventional method has the following problems.
The conventional smoothing processing is appropriately used for the electrophotographic system using one light beam. However, this method is inappropriate for a recording system such as an ink-jet system using a multi-nozzle, a thermal transfer system, and a sublimation system because the dot density is basically determined by the integration density of recording elements.
If a recording head having nozzles with a resolution of 300 DPI is used in an inclined state, the dot density can be increased to about 600 DPI. In this case, however, the dot density of the recording head is fixed at 600 DPI and cannot be returned to 300 DPI.
In a serial scanning system, when a recording frequency is raised in the scanning direction of a carriage, recording at a resolution of 600 DPI can be performed by nozzles at a pitch of 300 DPI only in the main scanning direction. In this case, a large smoothing effect can be obtained as for an oblique line having a gradient close to a direction perpendicular to the main scanning direction. However, few effect are obtained for an oblique line having a gradient close to the main scanning direction.
Assume that a circle is drawn with a thin line. In this case, as shown in FIG. 5 (to be described later), a portion improved by smoothing and a non-smoothed portion are simultaneously present, resulting in an unnatural circle.
If the dot density is to be increased in the main scanning direction of the carriage in the serial scanning system, and the maximum recording frequency of the recording head is kept unchanged, the speed of the carriage must be reduced to 1/2 or 1/4, resulting in a large decrease in recording speed. Even when the frequency can be increased, members with a large mass, i.e., the carriage or a head ink tank must be moved at a high speed. This causes a large decrease in energy efficiency and makes it difficult to efficiently perform smoothing as in a laser beam printer.